Let's say you've verified your info, first person you'd need to hire is an architect to draw up the plans and get it approved via NYC DOB.

I then had to have an architect due drawings to register all of this with the state.

Smith, RA - Principal Architect at Architect Owl PLLC (Licensed in NY & CT) 

Maybe the architect is thinking of something else? 

Sometimes the architects forget a requirement and you get kicked back to the start, sometimes your plans reviewer quits and they don't reassign you to a new one for a few months (that happened to me a while back), and sometimes the person spearheading this for you drops the ball.

These don't need to be architecturally-stamped so you can have a draftsman do it but you will likely need architectural plans for the permits if you get approval so it's often better to get architect involved early on.  

I've made absolute magic work to get this project done and need some real guidance, especially after I've learned throughout this process that most specialists (architects, engineers, etc.) only cover their exact box of scope and offer no additional help along the way.Each unit will easily rent for $2,200-2,500 per month so covering the loan after I'm done with construction is also not a concern. 

You'll need to ensure that your new parcels satisfy local regulations on size, shape, frontage, setback(depending on what you;ll build), work with your engineer and architect for the site plan for your building, parking, etc.Usually 2-6 months depending on your location and city approvals (or county).

If it seems like a good chance it could succeed, it would move to a longer feasibility period/contingency-on-variance and we would take the risk in hiring an attorney, potentially an engineer/architect, etc.

I have already spoken with a friend who's an architect/engineer and the lot can accommodate 4 to 5 additional units.